In Darkest Knight
by 800 words of heaven
Summary: In this issue, Millie and Sirius are back with another adventure! The task was simple: brew a potion that can turn people into man-bats - what could possibly go wrong? An outtake from Like a House on Fire Can be read alone Written for International Batman Day


"Langstrom Potion is a pretty dumb name for a potion," Sirius said. His thigh was flush with Millie's as they both peered into the cauldron bubbling away in front of them, the colour a deep purple. Of course, the bubbles themselves were bright green.

Soon, they'd be hearing maniacal laughter in the background, this potion was that crazy.

Millie shot him a quick glare as he pushed up closer to her, so that their arms brushed too. Sure, they were fully clothed and bundled against the always-present chill of the dungeons, but touch was touch.

"Have you ever heard of the concept of personal space?" she asked, keeping her eyes on the potion as she stirred it carefully.

"Course I have," he replied. "But I'm under the impression that potions, as inanimate objects, don't need it."

"You'll soon be an inanimate object if you jostle me one more time," she muttered. Sirius just grinned at his pretend-girlfriend and real-Valentine. Sure, Valentine's Day had been three weeks ago, but they hadn't exactly discussed changing the label on their established relationship. He hoped that Millie realised that he was as serious about her as her beloved superhero – whatever that actually meant – was about justice.

So in the meantime, until they both worked up the guts to bring up the change in the status quo, he'd refer to Millie White as his Valentine. It had a nice ring to it, really.

"That's three turns, by the way," Sirius noted. He was rubbish at Potions and even worse at Arithmancy, but even he could successfully count the number of stirs in a cauldron. "You wouldn't know that if I were lounging in my chair."

Millie graced him with another glare before replying, "I know how to count, Sirius."

Cheerily ignoring his Valentine's exasperation at him, he continued the string of his previous conversation. "So. Langstrom Potion. Terrible name. We must rename it."

She added a few leafy things to the cauldron, and stabbed them with the end of her spoon to drown them in the liquid. "Any suggestions?"

"Transmorgifying Potion."

"That's terrible. You need a more specific name."

"Transmorgifying Potion That Turns You into a Bat."

"Good luck fitting that all onto a label," she snorted. "Need something shorter."

"Bat Transmorgifying Potion."

"What's it with you and the word 'transmorgifying'?" she asked, blowing gently to send a bright green bubble veering towards the farther edge of the table. Her firewhiskey eyes followed its journey before it came to its popping end five feet away.

"It's a great word."

"It's a made-up word."

"It's a great word."

She sighed, a strand of her straight dark brown hair flying out of her face at the exhalation. Sirius was certain that it had sounded a little defeated. There was a part of his mind that wondered why she even voluntarily spent as much time as she did with him if she was always so put-upon, but that part of him was quickly drowned out by a voice crowing, "SHE'S MY VALENTINE. YAYAYAY!"

"Do you have any better ideas?" he asked, folding his arms across his chest. "Let's see you try to beat 'transmorgifying'!"

"That's quite simple," she replied, her voice prim and clear, one perfect eyebrow arched at him. Merlin, she was so hot when she got all holier-than-thou. "I could use a real word like 'transfiguration'."

After a heartbeat of silence, he ceded defeat. "Well-played, White. Well-played."

She gave him a grin which had something in the vicinity of his heart doing cartwheels. "Thank you. Also, how's Man-Bat Potion?"

"Hmm."

"Stop stroking your imaginary beard, Sirius."

As usual, he ignored Millie's suggestion and continued to stroke his imaginary beard in thought. "Not bad. Short, so it'll fit on the label. And catchy, so the apothecaries can market it with appropriately sensational advertising."

"Can you imagine it?" She leant back into the hard slats of her chair, the movement causing her skirt to move up half an inch where it brushed against his leg. Sirius tried very hard to concentrate on her voice rather than this new bit of exposed skin. "Take the Man-Bat Potion and you, too, can enjoy all that being a bat has to offer!"

"Human-sized leather wings!"

"Echolocation!"

"Being able to hang upside down without getting a headache!"

She laughed at the last one, and his stomach decided to join in on all the cartwheeling fun.

"They'd have to include a disclaimer though. Warn people against the sudden desire to move into a cave, and everyone asking if one of their parents was a vampire or something," Millie added very seriously, although her eyes twinkled.

"No wonder the potion is only used for educational purposes these days. Who even wants to be associated with a bat?"

"Hey!" she cried.

"Oh, I forgot. Your precious uber-hero." Sirius made a face. Merlin, it was sometimes difficult competing with the World's Greatest Detective. How did you even live up to that? And let's not forget the whole League of Shadows business. And the butler. Actually, the butler was the best part. So sassy.

(Okay, so he'd secretly read the comics. But only so he knew what he was up against for the battle for his Valentine's affections. Not that he enjoyed them or anything.)

"Superhero," she corrected automatically, her attention returning once more to the potion in front of them. The colours had inverted now, so that the liquid was green, and the bubbles purple. "Excellent," Millie muttered under her breath. "It's almost ready."

Sirius' eyes narrowed at the almost loving expression on her face, but before he could make fun of her for it, a loud _bang_ at the front of the room had both of them turning their heads to watch the commotion.

There was an upturned cauldron on the floor, the still-purple potion oozing across the slate tiles. Quiet sobbing was emanating from somewhere under a mop of straw-coloured hair – the face to whom the mop was attached was currently buried in their hands.

And Slughorn's sleeve was on fire.

Prongs and Evans in their capacity as Head Boy and Girl – and quite frankly, superheroes in their own right – were already up the front. Lily was swishing her wand in a very efficient-looking way, making the bungled potion vanish. A few more swishes, and the cauldron had returned to the table, and the area was cordoned off by a rudimentary magical barrier that shimmered green.

"The area will need to be scrubbed with a neutralising potion before that part of the floor is safe again," Millie murmured close to his ear. The hair on the back of his neck stood on end at the feeling of her breath washing over the shell. "Best to do it once the classroom empties."

 _Merlin's plaid kilt, Potions health and safety should not sound that sexy,_ Sirius thought.

He forced his attention back to the front, where Prongs had managed to douse the fire that was threatening to destroy Slughorn's florid green robes. It would have been a great service to the world of robe fashion if the ugly thing had been burnt to ashes. All the rotund Potions professor had needed to complete his "I'm a supervillain, here my evil laugh and cower" look was a bowler hat and an excess of question marks.

The class now saved from witnessing a painful origin story, Millie returned her attention to their potion, and Sirius returned his to admiring her with stars in his eyes.

"Well, I think our Man-Bat Potion is complete!" she declared.

Sirius obediently handed her the pre-labelled vial, unfortunately with its original moniker inscribed, and took the chance to surreptitiously sniff Millie's hair. Holy smokes, he loved the smell of her blueberry shampoo.

* * *

"Sirius, is it really necessary to sneak into an empty classroom for this?" Millie asked later that evening. They'd just left the library, having cut their Arithmancy study session short by a good fifteen minutes so that they could beat the rush of the closing-time exodus.

"It's too strange snogging in the library," Sirius replied, his cool hand holding her own. Their fingers were entwined as they walked down the deserted corridor. Millie's bag whacked her thigh with every step, and she used her free hand to adjust the strap. Sirius, of course, managed to look completely and utterly at ease with his own bag, the graceful arse. Sometimes, it was just so unfair.

Maybe that was his superpower.

"But the aquatic Herbology section is almost always deserted!" Millie replied. Okay, so they hadn't left the library early for practical reasons. But she figured that she shouldn't waste opportunities to lock lips with Hogwarts' self-proclaimed best snog.

(She had to secretly agree that Sirius, just this once, may have earnt his arrogance. He was a _fantastic_ snog.)

"That grumpy librarian has eyes everywhere!" Sirius cried, waving his arms around for effect, quite forgetting that one of those arms was attached to her own. She was pulled along by the movement, and managed to almost knock them both off their feet.

"She's not some sort of spy, Sirius!" Millie replied, although she did agree that Madam Pince was one scary lady. And she did have a certain knack of turning up to dust the _exact_ stacks in which you were playing tonsil hockey with your pretty-sure-we're-not-pretending-anymore-boyfriend.

"How do _you_ know that she's not part of ARGUS?"

"Because if she were a superspy from ARGUS she'd – wait a second." Millie's eyes narrowed. "How do you know what ARGUS is?"

Millie felt a certain satisfaction at watching colour rise up Sirius' face. Of course, he still looked like a model, whereas whenever she had the unfortunate luck of blushing, she resembled the Red Hood.

(She even had a tragic backstory to match, so it was a particularly fitting analogy.)

"I might have accidentally read all of your stupid comics," Sirius replied, rubbing the back of his neck and having the decency to look incredibly sheepish. Also incredibly adorable, like Ace the Bat-Hound, but she was trying to focus here.

"They're not stupid!" she retaliated.

"They so are! I mean, come on! He dresses up as a _bat_. And he lives in a _cave_. And he has _no_ imagination when it comes to naming his sidekicks!"

"He's rich! And a genius! And has a tragic backstory! And his butler is great! Also, it's _cute_ how he names his sidekicks!"

"But what about his gadgets? You have to agree that they have stupid names!"

They were no longer holding hands; instead, they stood in the middle of the corridor, facing each other, a good three feet of distance between them. Sirius' arms were folded across his chest, all superior defensiveness. Millie's hands were balled into fists at her side, her muscles locked to fight the temptation to reach for her wand and jinx the idiot for poking fun at the best superhero of all time.

She bristled with indignation on the Caped Crusader's behalf, but before she could shut down each and every one of Sirius' incredibly stupid arguments, a muffled roar reached their ears.

* * *

Sirius jumped about a foot in the air at the sound, immediately feeling like a complete and utter fool for it. He was saved from total embarrassment, because Millie appeared to have done the same. They were now standing next to each other once more, their argument over a dumbass fictional character with dumbass nicknames for everything momentarily forgotten.

Both their heads whipped towards the far end of the corridor, from where the sound came. Sirius tried to stare past the gloom of the last torch and see what was going on. Unconscious of even doing it, he realised that both he and Millie had taken a step towards the noise.

And that was when the glass window shattered inwards, and a large hulking shape flew through the newly created hole in their direction.

Sirius may have let out a yell of his own then.

Fortunately, the… _thing_ landed just beyond the main shrapnel from the window and stayed put.

Not so fortunately, his Valentine decided to go investigate.

"Millie!" he hissed as she took a few steps forward.

"What?" she hissed back, turning her head to glare at him with her usual exasperation.

"Where are you going?" He wasn't sure why he kept hissing, but it seemed important to keep their conversation (and fear) top secret, in case their unexpected cannonball had ears.

"To check on him!"

 _"Him?"_ Sirius gasped.

"Yes, him!" Millie replied as if it were the most obvious thing in the world. She rolled her eyes in his general direction before continuing on towards the damage at the far end.

Sirius stood there for a few moments in dumb shock as he watched the retreating back of his normally sane and rational pretend-but-not-really-girlfriend. Usually, _he_ was the one running recklessly into unknown situations, not a care for his own safety as he chased the next big thrill.

But here she was: Millie White in all her glory, her black cloak billowing out behind her because of the stiff March breeze whooshing through the window, gliding swiftly across the flagstone floor as if possibly-humanoid things smashed through windows on a daily basis.

Then again, given the usual goings-on in the (un)fair city of Gotham, this was just another Tuesday.

* * *

Millie reached the poor person on the floor just as Sirius decided to catch up to her.

"Wait for me!" he cried.

The person moaned, and curled even farther into a ball.

Millie whacked him on the arm and glared. "Quiet!" she snapped, although she felt much of the effect was lost since barely a whisper left her mouth. But Sirius, for once, managed to understand.

She took her wand out from her pocket, the tip illuminating itself at her silent command. Following her lead, Sirius took out his own wand, and they both crouched down in somewhat creepy synchrony.

She held her wand high to counteract their own cast shadows and couldn't help the gasp that escaped her lips at the sight.

The poor unfortunate soul who'd come flying through the window was completely nude, but that was the least of his problems.

No, probably one of his biggest problems would be that his arms had now morphed into giant batwings.

The resemblance to the flying nocturnal animal didn't end there, unfortunately. His ears, too, had changed. They were large and flapped out beside his head. His nose was looking distinctly snoutish, and she could see tiny pointy teeth poking out over his bottom lip.

Gesturing silently to Sirius to keep the light over their heads, she reached out as silently as she could, making sure to keep her movements as slow as possible so as not to startle the prone figure, to lift a single eyelid.

Just as she'd thought. The pupil had completely dilated to take over the iris and the whites. Like a bat's.

She moved back to rest on her heels for a moment, and placed a gentle hand on Sirius' arm to let him know that it was okay to lower it now. Thankfully, it was quite dark in this part of the corridor, and his eyes were closed, but night-adapted eyes could be quite sensitive to light (thank you, Care of Magical Creatures).

Sirius, seeming to understand the need for quiet, jerked his head back in the direction of where they'd been standing before, and Millie nodded her understanding.

Together, they got up from their positions on the floor, careful not to disturb the boy who'd seemed to have passed out amongst the shattered glass. They moved a little ways back, so that they could talk without hurting his ears.

"Tell me I'm not the only one seeing this," Sirius said, his voice hushed.

"No, you're not," Millie replied just as quietly, tapping her wand absently on her thigh.

"So that's really…"

"Yeah."

"Man-bat."

"Man-bat."

They both fell silent at that and turned to look back at the body, curled against the cold and almost-certain pain.

"I recognise that mop of hair," Sirius said into the growing silence.

Millie nodded. The straw hair was too distinctive to forget. "Some of the potion must have splashed onto his skin when it spilt over this afternoon," she said.

"But the results of the Langstrom Potion aren't usually so…"

"No, they're not. But it wasn't finished. It's actually the reason why it fell out of common use – it's too unstable and dangerous if not brewed correctly."

"Oh, and here I was thinking it was because no one really wants to be a bat," Sirius snorted.

"It doesn't make you a bat, it makes you bat- _like_ ," she corrected.

"If I wanted to be bat- _like_ , I'd dress like Snivellus."

Millie rolled her eyes, but didn't comment further. "We have to get him to the hospital wing."

"Right, of course." Sirius nodded.

Neither of them moved.

"Shall I levitate him?" he asked.

"No, I'll do it." She'd seen him do that spell, and quite frankly, she'd seen drunk people with better coordination than Sirius with a levitation charm.

They walked quietly back to the boy – Kirk – and with a swish-and-flick of her wand, she had him floating at chest level. He rotated slowly in the air, until he was on his back, his leathery wings flopped out beside him, the tips just touching the ground.

"Er… I think we should cover him up," Sirius murmured. His cheeks were faintly pink as he looked at her, and for some reason, heat creeped up her own. It wasn't as if she hadn't _seen_ a naked person before, but it felt as if she were deeply violating Kirk's privacy to be seeing him this way. He was hurt, and vulnerable, and wearing nothing but his birthday suit, and here she'd been, ready to levitate him like that _all the way_ to the Hospital Wing.

"Good idea," she croaked, her throat dry.

With an awkward bob of his head, Sirius undid the clasp at his neck and swished the cloak off his shoulders. Another swish, and it covered Kirk's body from neck to shin, the edges draping over his arm-wings.

"There you go, mate," Sirius murmured. "All warm and modest."

Millie raised her eyebrow at this uncharacteristic show of compassion, but didn't comment.

She twirled her wand as she turned away from the window. Behind her, Sirius muttered a quick _"Reparo!"_ and the glass shards shuddered and tinkled back into their pane. Luckily, the Hospital Wing was only a floor up and three corridors west from where they were, so it was a quick trip.

Madam Pomfrey only raised her eyebrows at the sight of Millie, Sirius, and a levitated man-bat.

"Langstrom Potion?" she asked, gesturing at a bed for Millie to deposit her charge.

Millie and Sirius nodded in unison. "He just came flying through the window!" Sirius supplied.

Madam Pomfrey sighed, flicking her wand once to send Sirius' cloak flying back into his face. Another had the curtains around the bed drawn for Kirk's privacy. "There's always one every year."

Millie and Sirius exchanged a look. There was a man-bat incident _every year_? Holy mother of Pennyworth.

"Will he be alright?" Millie asked.

"Yes, yes," the matron nodded absently. "I keep an antidote at hand during this time of year for cases like these. A few drops of that and a night of rest, and he'll be right as rain by morning."

Millie nodded.

"I'll be sure to mention your actions in helping your classmate to Professor McGonagall," Madam Pomfrey continued. "But it's best if you two headed back to your Common Room now, Miss White, Mr Black."

Millie just nodded again.

"Er… bye, Kirk!" Sirius called out.

They headed awkwardly for the door, and made their winding way back to Gryffindor Tower, subdued.

Halfway there, Sirius finally broke the silence. "We were quite the Dynamic Duo ourselves, there."

Millie couldn't help her smile, the idiot. "We were, weren't we?"

"Although I refuse to be the bloody bird of the pair." He sniffed as if the existence of a superhero named after a small fluttering thing was some sort of personal affront to him.

"So you'd rather be a bat?"

He shuddered at that. "You're right. Maybe it's better to be the bird."

"Why do you dislike him so much?" she asked. With Lily, she could understand. She had a thing for boys with glasses, so her preference for Metropolis over Gotham made sense. But Sirius? She'd thought that the World's Greatest Detective would be right up his alley.

Inexplicably, faint spots of pink flushed his cheekbones (yup. Still looked like a model though). "Isn't it obvious?" he muttered.

Millie shook her head. "No. Why in the name of Wayne would it be obvious?"

"Well, um…" His hand creeped to the back of his neck again, a sure sign that he was embarrassed about something.

"What is it, Sirius?" she asked, stopping in the middle of the corridor. It wasn't that important to know why Sirius seemed so offended by her favourite superhero in the grand scheme of things, but she was curious.

Sirius stopped too, and looked at the ground. Three… four beats of silence, then, "I'm jealous, okay?"

Millie blinked. "You're… jealous?" This was unbelievable.

"Yes."

"Of a fictional character."

"Yes." He lifted his head to look at her, and something about his grey gaze struck Millie anew, leaving her body feeling tingly in a distinctly we-should-snog-real-soon way.

"Explain."

"You're just always talking about him! And how amazing he is! And I'm just the guy who's conveniently here for a snog!"

"You're a bit high maintenance to keep around for just being a good snog, Sirius," Millie commented. Although he was a _very_ good snog.

"So you're saying that you'd snog him if he were real?"

"Of course I would! Just like you'd snog Celestina Warbeck if given the chance!" Yes, she knew _all_ about Sirius' little celebrity crush on the sultry songstress.

Sirius blinked. "It makes more sense when you put it that way," he muttered.

Millie rolled her eyes. Idiot boy.

She stepped towards him, placed her hands on his shoulders, reached on her tiptoes, and planted one right on his lips.

They parted in surprise, and Millie took full advantage of the opportunity to deepen the kiss. It took Sirius a few beats more to catch up, but when he did, he wrapped his arms around her waist, and tugged her closer.

They pulled apart a quite a few moments later, breathing deeply. She returned her heels to the ground, and loosened her grip from around his neck. Sirius, on the other hand, only tightened his hold, bringing their bodies impossibly closer.

"So… you _definitely_ like me more than him?" Sirius asked.

"You're ruining the moment, Sirius," Millie replied as she cuddled closer, wrapping her own arms around his waist.

"Sorry, but I just have to make sure."

She sighed. "Yes, I like you more than him."

"Even though he's Gotham's finest?"

"That would be Jim Gordon and his pals over at the GCPD, Sirius."

"Oh, right."

"But yes, even though he's a billionaire-genius-playboy by day, and a crime-fighting badass with a cool cape by night, I like you more. Because you're _real._ "

"For the record, I'd be the better snog, too."

Millie laughed. "Also the bigger idiot. And that's why you're the Robin of our Dynamic Duo."

"You're probably right there."

They cuddled for a few more minutes in the middle of the corridor, before once more continuing on to the Common Room.

"Who do you think would be Robin out of Prongs and Evans?" Sirius asked. "No, wait, I know. Prongs, obviously."

Millie smiled. "Obviously."

* * *

 **AN: Hello lovely readers! Happy International Batman Day! This is an outtake of sorts from _Like a House on Fire_ , but can hopefully be read as a standalone. It slots in right after chapter 31, though. Batman and Gotham were created by Bob Kane and Bill Finger; the Joker, Robin, and Alfred Pennyworth were created by Bill Finger, Bob Kane, and Jerry Robinson; Man-Bat was created by Frank Robbins and Neal Adams; the League of Shadows was created by Denny O'Neil and Neal Adams; the Riddler was created by Bill Finger and Dick Sprang; ARGUS was created by Geoff Johns and Gene Ha; Ace the Bat-Hound was created by Bill Finger and Sheldon Moldoff; Superman and Metropolis were created by Jerry Siegel and Joe Shuster; and Jim Gordon was created by Bill Finger, Lawrence Donovan, Henry Ralston, and John Nanovik. All of these (and any other references I missed) are owned by DC Comics.**


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